Short-range wireless communication techniques have been used in various fields to transmit and receive data in a contactless manner. Radio frequency identification (RFID) is a widespread, typical short-range wireless communication technique. A common RFID system includes an RF tag attached to a target object (hereafter, a workpiece) or an object such as a pallet or a container supporting or containing the target object, an interrogation device (hereafter, a reader/writer) that transmits and receives data to and from the RF tag, and a host device that controls the reader/writer.
An RFID system that uses an ultrahigh frequency (UHF) band has a relatively large range of interrogation distances of about 1 to 5 m between a reader/writer and an RF tag. This often causes the reader/writer to interrogate an RF tag that is not a target RF tag to be interrogated (hereafter, an unintended RF tag). To prevent this, the reader/writer is typically set to change the transmission power from its antenna and to adjust the range of its interrogation distance. This method optimizes (minimizes) the transmission power and thus prevents the reader/writer from interrogating an unintended RF tag.
However, adjusting the transmission power has been difficult for users without highly technical knowledge.
At an actual site where an RFID system is installed, interrogation signals from different reader/writers can interfere with one another depending on the distance between such reader/writers or depending on the number of reader/writers installed. A reader/writer installed at too large an interrogation distance may read an unintended RF tag located within the distance.
In response to these, Patent Literature 1 (Japanese Patent No. 5798599) describes a technique for adjusting the transmission power in accordance with the level of received power from an RF tag. Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2000-49699) describes a technique for first setting a maximum radio signal output level and then gradually lowering the output level while transmitting data to a peripheral device, and determining a minimum output level at which no error is detected in a response signal from the peripheral device. Patent Literature 3 (Japanese Unexamined Patent Application Publication No. 2002-170082) describes a method for switching the radio transmission power in accordance with the strength of a signal received from an integrated circuit (IC) card. In switching the transmission power, the degree of modulation is also changed in cooperation with the switching of the gain for amplifying the transmission power.